The 2'-5' oligoadenylate synthetases (OAS) are a family of enzymes which play an important role in the mammalian innate immune system by conferring resistance to viral infections. Upon interferon stimulation of cells, latent OAS is produced and subsequently activated by double-stranded RNA. Active OAS produces 2'-5' linked oligoadenosines which in turn dimerize and activate RNase L, an endoribonuclease that degrades cellular and viral RNA. Structural studies of OAS are valuable since they will provide insight into the mechanisms for the OAS 2'-specific nucleotidyl transferase reaction, and for the RNA activation of the enzyme. We are in the process of completing the first crystal structure determination of an OAS protein, that of a latent enzyme without bound substrate or activating RNA. This structure reveals a structural similarity with 3'-specific polymerases. Analysis of this structure provides a basis for designing mutagenesis experiments to test mechanistic hypotheses, and for selecting RNA constructs for continuing structural studies. [unreadable] [unreadable] From the comparison between the OAS active site and those in 3'-specific polymerases, we hypothesize that the mechanisms for the 2' and 3' nucleotidyl transferase reactions are similar, and that the 2' specificity may arise from a differing position of the substrate. Specific Aim 1 is to investigate the components of the OAS catalytic machinery which are responsible for the unique 2' specificity of its nucleotidyl transferase reaction. This will be done by observing the functional consequences of designed mutants in which proposed active site amino acid residues have been substituted, and by pursuing crystal structures of OAS bound to ATP substrate, 2-5A substrate/product, or analogs. These efforts constitute a small, self-contained project which can be carried out with modest resources. From the analysis of our apo OAS crystal structure, we hypothesize that OAS activation requires a conformational change of the protein which likely occurs upon RNA binding. Specific Aim 2 is to investigate the mechanism of OAS recognition of viral RNA, and of the subsequent activation of the OAS enzyme by double-stranded RNA. Crystals of complexes of OAS bound to activating double-stranded RNA, or to non-activating single-stranded RNA, will be pursued. These crystallization experiments for OAS-RNA complexes are feasibility studies for the development of the OAS project, which is a new direction for the Principal Investigator's laboratory. [unreadable] [unreadable]